This pilot project will define the innate immune pathways critical for host response to a Category A biodefense pathogen, Rift Valley Fever Virus (RVFV), by elaborating molecular components of early cellular responses to RVFV in vitro, using established human-derived transformed cell lines and murine cells from specific gene knockout mice. In semi-arid regions of Africa and the Arabian Peninsula, natural epizootics of RVFV occur every ~8 years among wildlife and domestic animals. These outbreaks are associated with high mortality among affected livestock, and are usually associated with simultaneous epidemics of RVFV-associated human disease. In such settings, transmission to humans occurs either by insect bite or aerosol inhalation, which occurs during exposure to body fluids of RVFV-infected animals. Based on recent work in Kenya, we have determined that the extent of RVFV transmission to humans is much greater than previously suspected, both during epidemics and inter-epidemic periods, with up to 25% lifetime risk of RVFV infection in threatened human communities. Innate immunity to RVFV has not been extensively characterized. In animal models of RVFV infection, a strong protective role has been identified for early type I interferon (IFN) responses. In addition, in human infection, a delayed onset of IFN response is associated with the more severe forms of RVFV-induced clinical disease. Toll-like receptors (TLRs) constitute a class of membrane bound, germ-line encoded pattern-recognition receptors (PRRs) capable of detecting viral particles or products of viral replication, and stimulating early IFN responses. Another class of cytoplasmic molecules has recently been implicated in virus-induced IFN gene regulation. Retinoic acid-inducible gene-I (RIG-I) and melanoma differentiation-associated gene 5 (Mda-5)/Helicard), two DExD/H box RNA helicases contain protein interaction caspase recruitment and activation domains (CARD) and recognize specific viral products. We propose to identify critical PRRs and signaling pathways involved in the innate response to Rift Valley Fever virus. We hypothesize that interactions of RVFV with TLRs and/or RIG-I and Mda-5 are critical for the induction of type I IFNs as part of the early protective and/or disease-mediating immune responses that occur during the course of human infection. Specifically our aims are to 1) to define the relative role TLRs or RNA helicase respective common adaptor signaling in RVFV replication and innate immune activation, 2) to define specific TLRs and/or helicases involved in RVFV induced type I IFN and inflammatory cytokine responses. Novel results from the proposed studies will be used to support development of a larger, population-based project to be submitted to NIAID for extramural funding. PUBLIC HEALTH RELEVANCE We proposed highly relevant studies to define the innate immune receptors and pathways involved in early host defense against Rift Valley fever Virus. Rift Valley Fever virus (RVFV), a Phlebovirus in the Bunyaviridae family, is designated a Category A biodefense pathogen based upon its projected severe impact on public health and agriculture in North America in the event of a deliberate release. In semi-arid regions of Africa and the Arabian Peninsula, natural epizootics of RVFV occur every ~8 years among wildlife and domestic animals. These outbreaks are associated with high mortality among affected livestock, and are usually associated with simultaneous epidemics of RVFV-associated human disease. Based on recent work in Kenya, we have determined that the extent of RVFV transmission to humans is much greater than previously suspected, both during epidemics and inter-epidemic periods, with up to 25% lifetime risk of RVFV infection in threatened human communities.